The present invention relates to a duplexer and a transceiver used in a bidirectional communication apparatus using a radio wave.
In the field of radio communication using radio waves, there is the so-called frequency division duplex communication in which different frequencies are used for transmission and reception. A station that uses one antenna to accomplish bidirectional communication has a duplexer to prevent the signal transmitted by the station from directly entering the circuit for receiving signals from other stations. In general, the frequency characteristics of the duplexer cannot be changed. Therefore, a communication apparatus capable of using a plurality of frequency bands has a plurality of duplexers and switches among the duplexers in order to cover the plurality of frequency bands (see Masaaki Koiwa, Fumiyoshi Inoue and Takashi Okada, “Multiband Mobile Terminals”, NTT DoCoMo Technical Journal, Vol. 14, No. 2, pp. 31-37, July 2006).
Conventional approaches have a problem that the circuit area and the number of components increase as the number of frequency bands increases. In general, the duplexer has a filter that permits a signal at the transmission frequency to pass therethrough and reflects a signal at the other frequencies and a filter that permits a signal at the reception frequency to pass therethrough and reflects a signal at the other frequencies. Alternatively, a duplexer capable of changing the frequency characteristics can be used, and the frequency characteristics can be appropriately changed. However, in the typical frequency division duplex communication, the transmission frequency and the reception frequency are relatively close to each other, and therefore, the filters have to have a narrow frequency band. In order for the filters to have a narrow band (or in order to bring the transmission zero close to the resonant frequency), the filters have to have a plurality of resonators. Thus, there remains the problem that the circuit area and the number of component increase.
The present invention has been made in view of such circumstances, and an object of the present invention is to provide a duplexer that functions as a filter having variable frequency characteristics and is reduced in circuit area and number of components and a small and lightweight transceiver.